Cutting tools having single- or multi-layered hard coatings of TiAlN, TiC, TiN, Ti(CN), Al2O3, etc. are conventionally used to cut heat-resistant alloy steel, stainless steel, etc. With increasingly severer use conditions of such hard-coated tools, cutting tools undergo extremely high cutting edge temperatures during, for example, high-speed cutting of soft steel. At cutting edges subjected to high temperatures, the crystal structures of the hard coatings are changed to lower hardness, suffering crater wear on rake faces and thus shorter lives. To overcome such problems, cutting tools having hard coatings with better wear resistance and oxidation resistance at high temperatures are desired.
JP 2001-341008 A discloses a cutting tool having a hard titanium aluminum nitride coating containing 0.01-2% by mass of chlorine and having an fcc structure, the coating being formed on a WC-based cemented carbide substrate by a thermal CVD method at 700-900° C., using a titanium halide gas, an aluminum halide gas and an NH3 gas as starting material gases. It has been found, however, that because the hard titanium aluminum nitride coating of JP 2001-341008 A has a fine granular crystal structure, it exhibits low oxidation resistance when used at high temperatures, resulting in a short life.
JP 2008-545063 A discloses a hard-coated tool having a hard titanium aluminum nitride coating having an fcc structure with a lattice constant of 0.412-0.405 nm, which has a composition represented by Ti1-xAlxN (0.75<x≤0.93) and is formed by a thermal CVD method on a substrate, or a multi-phase coating comprising a main phase of titanium aluminum nitride and other phases. It has been found, however, that because the hard titanium aluminum nitride coating of JP 2008-545063 A also has a granular crystal structure, it exhibits low oxidation resistance when used at high temperatures, resulting in a short life.
JP 2014-129562 A discloses a tool covered with a hard titanium aluminum nitride coating having a multi-layer structure by a CVD apparatus 100 shown in FIG. 14. The CVD apparatus 100 comprises pluralities of shelves 103, on which pluralities of substrates 102 are set, a reaction vessel 104 covering the shelves 103, a temperature-controlling cover 105 surrounding the reaction vessel 104, a supply pipe 108 having two inlets 106, 107, and a discharge pipe 109. The hard titanium aluminum nitride coating has a structure in which first and second unit layers having hard grains of TiAlN, AlN or TiN are alternately laminated on a WC-based cemented carbide substrate. This is because (1) first and second starting material gases (mixture gases) are ejected into the furnace through nozzles positioned at equal distance from a center of the supply pipe 108 in just opposite directions (180°); and because (2) different starting material gases from those in the present invention are used. It has been found that the hard titanium aluminum nitride coating having a laminate structure suffers interlayer delamination at high temperatures because of thermal expansion coefficient difference between the layers due to composition difference; and that it exhibits largely decreased oxidation resistance because of a fine crystal grain structure when used at high temperatures, resulting in a short life.